1. Field of the Invention
The present invention relates to a cathode ray tube (CRT) used in a color television or a high-definition monitor television and further to an electron gun used in an electron beam exposure device or the like that utilizes a converged electron beam. In particular, the present invention relates to a field-emission electron source element used in an electron gun of a highly bright CRT requiring a high current density operation, and an image display apparatus using the same.
2. Description of Related Art
In recent years, with the advent of thin-type displays such as liquid crystal displays or plasma displays, the flat display market has been growing rapidly, though CRT displays still hold an edge in price and performance in application to home televisions about 32 inch diagonal in size. Also, when the digital terrestrial broadcasting is newly introduced, a display technology for television is expected to change drastically. While the television systems are shifting toward a digital system, there is a strong demand for a high-resolution performance of a display, in particular.
However, the television technology that has been used widely so far might not be able to respond to such a demand sufficiently. An electron gun is used in a television as a core portion for displaying an image, and its performance is closely related to the resolution performance. By increasing a current density of a cathode used in the electron gun, it becomes possible to reduce an effective area of the cathode, thereby improving the resolution performance. Although various technological improvements on a hot cathode material that is currently used as the cathode of the electron gun have been made to increase the current density, such improvements have come close to their physical limits and no more dramatic increase in the current density can be expected. A cathode in an electron gun for digital broadcasting, which has been proceeding toward a practical use in recent years, requires about 6 to 10 times as large a current density as a conventional hot cathode. Accordingly, there are increasing expectations for a cold cathode as a technology for achieving a considerable increase in the current density.
On the other hand, an idea of using the cold cathode in the electron gun has been suggested conventionally. The cold cathode has a feature in which a high-density mounting of minute cathodes allows a higher current density. Accordingly, the cold cathode has been commercialized in some products such as electron microscopes.
As the first suggestion of using the cold cathode in a CRT, a color picture tube using a field-mission cathode is disclosed in JP 48(1973)-90467 A. Using the field-emission cathode in the color picture tube is advantageous not only in increasing the current density as described above but also in lowering power consumption. The conventional hot cathode system has required a heater for heating to emit electrons and, thus, consumed electric power of about several watts even in a standby state where the electron gun is not in use. On the other hand, the field-emission cathode, which requires no heater, has the advantages that not only is the electric power not wasted during standby but also the electron gun is activated instantaneously.
In general, a high-melting metal such as molybdenum often is used as a material for the cold cathode. After the completion of CRT manufacturing process, the degree of vacuum inside the CRT usually is about 10−4 Pa owing to constraints in the manufacturing processes and the structure of the CRT. When the cold cathode is operated at a current density of about 10 A/cm2 under such a vacuum environment, the following problem arises. Inside the CRT, there are various kinds of residual gases that have been generated in the manufacturing process. It is known that oxygen (O) and carbon (C) among the constituent elements of the residual gases temporarily adhere to an emitter surface or change a composition of the emitter surface, thereby lowering an emission performance of the cold cathode.
In response to the above problem, JP 2000-36242 A discloses that the stabilization of an emission current is achieved by utilizing a hydrogen gas (H2). In the following, this will be described with reference to FIG. 8 showing a cross-section of a field-emission light-emitting element using a conventional field-emission electron source element.
A cathode conductor 3 is formed on an upper surface of a cathode substrate 2 of a field-emission light-emitting element 1, and an insulating layer 4 is formed on the cathode conductor 3. On the insulating layer 4, a gate 5 is formed, which contains hydrogen absorbing metals such as Nb, Zr, V, Fe, Ta, Ni and Ti. A plurality of openings 6 are formed in the gate 5 and the insulating layer 4 so as to extend continuously in the thickness direction. On the cathode conductor 3 that is exposed to bottoms of the openings 6 of the insulating layer 4, emitters 7 are formed. Further, an anode conductor 9 is formed on an inner surface of an anode substrate 8, and a phosphor layer 10 is formed on the anode conductor 9.
When the field-emission light-emitting element 1 is turned on, a driving signal is supplied to the anode conductor 9, an intersection of a matrix is selected by the cathode conductor 3 and the gate 5, and the phosphor layer 10 corresponding to a desired position of the anode conductor 9 is made to emit light. The anode conductor current is monitored constantly, and when it falls below a certain level, a signal is supplied to the gate 5 during non-lighting period. In this manner, if an electron hits the gate 5, hydrogen and methane (CH4) are emitted to the vicinity of the emitters 7, thus removing oxygen and carbon adhering to the emitters 7. This prevents an increase in a work function of the emitters 7, thereby restoring the emission performance. As a result, long life and high reliability of the emitters 7 can be achieved.
In this conventional example, by allowing the electron to hit the gate 5 containing the hydrogen absorbing metals, oxygen and carbon adhering temporarily to the emitters 7 are removed, thereby restoring the emission performance. In the case where a high-melting metal such as molybdenum is used as the material for the emitters, since a chemical bond between the high-melting metal on the surface of the emitter 7 and the oxygen and carbon adhering thereto is weak, it is relatively easy to remove the oxygen and carbon by the method according to this conventional example.
However, in the case where other materials, for example, silicon and the like are used as the material for the emitters, there is a problem that the emission performance cannot be restored by using the above-described method of the conventional example. In general, the outermost surface of a clean silicon serving as the emitter is chemically unstable because dangling bonds of the silicon are not terminated. In this case, even a slight amount of oxidizing gases such as H2O and CO2 present in the vicinity of the emitter causes oxygen in these oxidizing gases to form a bond easily with the dangling bonds in the silicon surface, so that an SiO2 film is formed on the emitter surface. Since the SiO2 film on the emitter surface lowers the electron emission performance of the emitters, the emission current drops considerably. Furthermore, since the SiO2 film is extremely chemically stable, there arises a large problem that once the Si—O bonds are formed in the emitter surface, it is extremely difficult to remove them even by using the gases such as hydrogen and methane in the conventional example, so that the emission performance cannot be restored. Therefore, in the case of using as the emitter a material that is easily oxidized and forms a stable oxide film, such as silicon, it is crucial not to allow the formation of the oxide film that deteriorates the emission performance on the emitter surface.